. The study of animal life . ts and animals alike—were built up ofcells, cellular biology may be said to date. It was soonshown as a corollary that ever> organism which reproducedin the ordinary fashion arose from a single egg-cell orovum which had been fertilised by union with a male-cellor spermatozoon. Moreover, the position of the simplestanimals and plants was more clearly appreciated ; they aresingle cells, the higher organisms are multicellular. Now the cells of the animai body are necessarily varied,for the existence of a body involves division of labour CHAP. XI TJie Elements of St
. The study of animal life . ts and animals alike—were built up ofcells, cellular biology may be said to date. It was soonshown as a corollary that ever> organism which reproducedin the ordinary fashion arose from a single egg-cell orovum which had been fertilised by union with a male-cellor spermatozoon. Moreover, the position of the simplestanimals and plants was more clearly appreciated ; they aresingle cells, the higher organisms are multicellular. Now the cells of the animai body are necessarily varied,for the existence of a body involves division of labour CHAP. XI TJie Elements of Structure i8i among the units. Some, such as the lashed cells liningthe windpipe, are very active, like the Infusorian Protozoa ;others, for instance the fat-cells and gristle-cells of connectivetissue, are very passive, something like the Gregarines;others, such as the white blood corpuscles or leucocytes,are between these extremes, and resemble the amoeboidProtozoa. But it is true of most of them that they consist (i) of a. Fig. 34.—Animal cell, showing the coiled chromatin threads of the nucleus (<t),and the protoplasmic network (i5) round about. (From Evolution of Sex \after Carnoy.) complex, and in part living cell-substance, in which keeneyes looking through good microscopes detect an intricatenetwork, or sometimes the appearance of a fine foam ; (2)of a central kernel or nucleus, which plays an importantbut hardly definable part in the life of the cell, especiallyduring the process of cell-division ; (3) of a slight outermembrane, varying in definiteness and sometimesquite absent, through which communications with neigh- 182 The Study of Animal Life part hi bouring cells are often established; and (4) of cell contents,which can be chemically analysed, and which are productsof the vital activity rather than parts of the living substance,such as pigment, fat, and glycogen or animal starch. The growth of all multicellular animals depends upon amultiplication of t
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